Jenkins docker Error: java.io.FileNotFoundException

Question

I am trying to use docker agent in Jenkinsfile:

pipeline {
    agent {
        docker {
            image "openjdk:jdk-alpine"
        }
    }

    stages {
        stage("Hello") {
            steps {
                sh "echo 'Hello'"
            }
        }
    }
}

But I have got:

/var/jenkins_home/workspace/tesler@tmp/durable-7aaf5de1/script.sh: 1: /var/jenkins_home/workspace/tesler@tmp/durable-7aaf5de1/script.sh: docker: not found
[Pipeline] isUnix
[Pipeline] sh
+ docker pull openjdk:jdk-alpine
/var/jenkins_home/workspace/tesler@tmp/durable-af6a5d64/script.sh: 1: /var/jenkins_home/workspace/tesler@tmp/durable-af6a5d64/script.sh: docker: not found

I am testing this with Jenkins casc. I need docker so I have copy its definition to casc file from https://github.com/jenkinsci/configuration-as-code-plugin/tree/master/demos/docker It is not working. In jenkins dockers lists there is an error:

I hope that when I will fix docker error Jenkinsfile will work too. What is wrong?